Don't cache default callable return value of None in cache.get_or_set()

[Dan Tao]

In ​82be474 the change was made to allow None as a default value passed to
BaseCache.get_or_set. In case the default value of None is used, it is not
stored in the cache. This still left open the possibility that default could
be a callable that returns None, in which case it would be stored in the
cache.

# This scenario works as expected.
cache.get_or_set('foo', None)  # None
cache.get_or_set('foo', 5)  # 5
cache.get('foo')  # 5

# This scenario seems wrong.
cache.get_or_set('bar', lambda: None)  # None
cache.get_or_set('bar', 5)  # None :(
cache.get('bar')  # None :(

[Dan Tao]

I have already created a pull request for this issue, but I'm not sure how to officially associate it with this ticket so I'll just link to it here in a comment:

​https://github.com/django/django/pull/9087

[Tim Graham <timograham@…>]

In 4d60261b:

Fixed #28601 -- Prevented cache.get_or_set() from caching None if default is a callable that returns None.

[Tim Graham <timograham@…>]

In dc112bf5:

[2.0.x] Fixed #28601 -- Prevented cache.get_or_set() from caching None if default is a callable that returns None.

Backport of 4d60261b2a77460b4c127c3d832518b95e11a0ac from master

[Tim Graham <timograham@…>]

In 45b0ec8:

[1.11.x] Fixed #28601 -- Prevented cache.get_or_set() from caching None if default is a callable that returns None.

Backport of 4d60261b2a77460b4c127c3d832518b95e11a0ac from master